Square room, walls to bend to create round celling angles
Depending a bit on your other requirements, perhaps the simplest way would be to reduce the corners to a cube. and then CtrlShiftB bevel the vertices at the corners.



If you want to leave yourself some flexibility, you could do this with a Bevel Modifier, set to 'Only Vertices' and 'Weight' adjusting the Vertex Bevel Weight of the corners you want rounded, and the profile, segments, etc., whenever you like...